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An assessment of the spatial variability of biophysical factors near a CO2 flux tower in a deciduous forest and its influence on annual estimates of net ecosystem exchange.
SCHMID, HAPE1, OLIPHANT, ANDREW1, WAYSON, CRAIG*,1, RANDOLPH, JC1, 1 INDIANA UNIVERSITY, BLOOMINGTON
ABSTRACT- All natural forest ecosystems exhibit spatial variability at some range of scales. Micrometeorological measurements of CO2 exchange are thus only expected to be representative of the forest to the extent that the biophysical forcings in the flux footprint reflect average forest conditions. Here, we examine the influence of spatial variability in biomass (as expressed by the normalized difference vegetation index, NDVI, or LAI derived from it), and radiative exposure (through variations in terrain slope angle and orientation) on net ecosystem exchange of CO2. A high resolution IKONOS satellite scene is used to determine the distribution of NDVI in the vicinity of an AmeriFlux tower site at Morgan-Monroe State Forest in Indiana, USA. Slope angle information obtained from a digital terrain model of the area is used to derive APAR from LAI and the modeled PAR distribution. The spatial representativeness of CO2 flux measurements at 46 m (1.8 canopy heights) above the forest is assessed by comparing the weighted distribution of LAI contained in the flux footprint to the average LAI of the forest. The same analysis is also doen for APAR. An analytical, three-dimensional footprint model is used to estimate the source weight distribution for this procedure. Source weight distributions for unstable, neutral and stable conditions are overlaid and multiplied with the LAI and APAR distributions for the stabilities and wind directions measured in 1999. The result is a quantitative distribution of representativeness, depending on wind direction and stability.
KEY WORDS: flux bias